1. Field of the Invention
This invention relates to radio transmission systems in which a plurality of transmitters intermittently transmit short messages indicative of status of sensors associated with the transmitters and to a method of synchronization suitable for using frequency hopping carrier in such systems.
2. Description of the Prior Art
In certain types of radio transmitter systems there exist many transmitters that periodically transmit very short messages to one or more receivers. One example of such systems is burglary and fire alarm systems. In these systems, many transmitters located at different places in a building transmit messages indicative of the status of monitoring sensors to a receiver that collects the data from the sensors. Normally, the transmitters transmit supervisory status messages that are as short as feasible and the period between the transmissions is as long as feasible in order to minimize the average current drain as the transmitters in these systems are typically battery operated. In addition, short and infrequent transmissions lower the probability that the data is lost due to collisions which occur when two or more transmitters transmit at the same time. However, when an alarm condition occurs, a transmitter transmits immediately in order to convey the alarm message with little delay.
Typically, such systems transmit data at a single frequency. Thus, they are susceptible to interference and signal loss due to phenomena known as multipath fading. Consequently, the reliability of such systems is compromised or conversely, the transmitted power has to be increased to overcome the fading which results in larger power drain and shorter battery life. Besides, there usually are regulatory limits that restrict such transmitter power and thus limit the possible compensation by sheer increase of power. Since the multipath effect is highly sensitive to the frequency of the transmitted carrier, the frequency hopping spread spectrum technique has a potential to eliminate these drawbacks. However, frequency hopping systems require long acquisition time and they are typically used in two way communication applications in which all the devices are continuously synchronizing with one master device or with each other using a variety of synchronization methods as shown in some of the following references. In other cases, to ease the synchronization problem, there are employed multichannel receivers that can simultaneously receive signals at many frequencies by making the receiver broadband or by using several receivers at the same time. Generally, those solutions suffer from performance degradation or high cost or both which makes them less desirable for low cost applications that require high reliability such as security alarms.
For example according to U.S. Pat. No. 4,843,638 granted to Walters, a receiver local oscillator has a comb spectrum. This effectively makes it a wide band since each of the frequency range down-converted by the spectral components of the local oscillator will fall in the receiver pass-band. Consequently, the sensitivity of such receiver will be adversely affected.
In another example according to U.S. Pat. No. 5,428,602 granted to Kemppainen, each hopping frequency is monitored by a separate receiver. This is very costly and presently not suitable for low cost systems.
In another example according to U.S. Pat. No. 4,614,945 granted to Brunius et al., a system is described that allows multiple instruments to be monitored and data to be simultaneously transmitted by several radio transponders. However, in order to operate properly, the transponders have to be energized by a RF signal to begin a transmission sequence. This necessitates a radio receiver to be included in the transponder. This makes the system a two-way communications system. Such systems are inherently more complex and costly than one-way communications systems.
In yet another example according to U.S. Pat. No. 5,659,303 granted to Adair, a transmitting apparatus is described that transmits bursts of data continuously at varying time intervals and at varying frequencies. However, the apparatus as described in the preferred embodiment and associated claims, allows identical hopping pattern to be realized in the transmitters. A means is provided to offset the starting point of the variation sequence for different transmitters depending on the transmitter ID, so that the hopping sequences in various transmitters are initially offset in respect to each other. However, due to unavoidable reference frequency drifts that are different in various transmitters, the sequences may slide in respect to each other. Consequently, it is only a matter of time that the patterns of two or more transmitters will be aligned thus producing a condition for persistent collisions of the transmitted data bursts. In addition, Adair does not provide for a receiving apparatus or a method that would allow such transmitted signals with varying frequencies to be received. In the case of Adair""s invention, the actual sequence used by a transmitter is not predetermined but instead it may vary with temperature and depends on the transmitter circuit design and manufacturing tolerances, therefore the signal acquisition is made even more difficult.
A serious problem that must be addressed in battery operated systems is to shorten the transmission time as much as possible by making the message preamble as short as possible in order to conserve the battery power. Therefore, the synchronization of the receiver with the transmitters is a difficult task. This problem is exacerbated in some systems such as security alarms that require some messages to be conveyed to the system immediately without waiting for the scheduled transmission time. A related problem in battery operated systems is limitation of the transmitted power to conserve the battery power. The frequency hopping system, if designed properly, can be advantageously used to combat multipath fading that is a major source of transmitted signal attenuation. Consequently, proper method and construction of the receiver is of great importance. The system design and the receiver design should be done to support each other advantageously.
For example according to U.S. Pat. No. 5,428,637 granted to Oliva, et al., a method is described to reduce the synchronization overhead in frequency hopping systems to reduce the burden of re-synchronization before each separate transmission. The method is based on allocation of specific time slots for any unit that desires to transmit data and thus the method requires a two-way communications to accomplish the necessary exchange of series of reservation and acknowledge messages.
Similarly, in yet another example according to U.S. Pat. No. 5,438,329 granted to Gastouniotis et al., a two-way system is used for efficient operation of a telemetry system that is designed to allow operation in the presence of multipath fading and interference.
Book References:
Robert Dixon, xe2x80x9cSpread Spectrum Systemsxe2x80x9d, John Wiley and Sons, 1884, ISBN 0-471-88309-3.
Marvin K. Simon et al, xe2x80x9cSpread Spectrum Communications, vol. 1,2,3xe2x80x9d, Computer Science Press, 1985, ISBN 0-88175-017-4.
Don J. Torrieri, xe2x80x9cPrinciples of Secure Communication Systemsxe2x80x9d, Artech House, 1985, ISBN 0-89006-139-4.
Accordingly, it is an object of this invention to provide a radio transmission system including many radio transmitters that use frequency hopping carrier to intermittently transmit very short messages indicative of status of sensors associated with the transmitters, and to provide a synchronization means and method that allows a frequency hopping receiver to acquire and to maintain synchronization simultaneously with all the transmitters, thus, relieving transmitters from transmitting a long preamble each time a message is transmitted that may otherwise be required if an acquisition is performed separately for each message and which would result in an excessive current drain and shortened battery life in battery operated transmitters.
It is another object of this invention to provide a method of transmission in such a system so as to improve reliability of the system in the presence of multipath fading and interference.
It is a further object of this invention to provide a method of eliminating the effect of persistent collisions in such a system that occur when two or more transmitters transmit at the same time and at the same frequency for a prolonged period of time.
It is still a further object of this invention to provide a method that allows such a system to convey the information about an abnormal sensor condition as soon as the condition occurs regardless of the transmission period of the associated transmitter.
It is still a further object of this invention to provide a method of transmission resistant to a deliberate interference and having a means to differentiate between valid and fraudulent transmissions.
According to one aspect of the invention there is provided a frequency hopping radio transmission system comprising a plurality of transmitters and associated sensors and a receiver wherein said transmitters intermittently transmit very short messages indicative of status of the sensors associated with the transmitters, wherein (1) each transmitter includes a time interval generator means to produce pulses controlling the time interval between successive transmissions, a frequency synthesizer-modulator means to generate a modulated radio frequency carrier signal wherein the frequency of the carrier changes in response to programming the synthesizer by digital data, a reference frequency oscillator providing a frequency reference from which the synthesizer derives carrier frequencies and, preferably, from which the time interval generator derives its timing, a transmitter control logic means activated in response to pulses from the time interval generator or a sensor signal indicating an abnormal condition, wherein when activated, the transmitter control logic activates and programs the synthesizer so that the transmitter carrier frequency is changed according to a predetermined frequency hopping algorithm, provides digital data indicative of the sensor status and preferably battery status, and modulates the carrier with the provided data; (2) the receiver includes a frequency selective radio receiver circuit, programmable by digital data, to receive and demodulate a transmitted carrier when the frequency of the receiver circuit is programmed according to the frequency of the carrier, and a receiver control logic means to process demodulated data, to provide system interface responsive to the received data, and to program the frequency of the frequency selective receiver circuit, wherein the control logic includes a receiver timer to measure the elapsing time, and a plurality of memory registers to hold digital data indicative of (a) the time of the next transmission occurrence for each transmitter and (b) the frequency of the next transmission occurrence for each transmitter, wherein in operation, the control logic sequentially compares the data content of the time registers with the data content of the timer and if the transmission is due from a transmitter, the control logic programs the frequency selective radio receiver circuit according to the data content in the frequency register associated with said transmitter, attempts to decode the demodulated signal, modifies the content of the time register by a number representative of the time interval between the successive transmissions for said transmitter and modifies the content of the frequency register according to a predetermined algorithm for said transmitter.
According to the second aspect of the invention, there is provided a method of transmission in the system so as to improve reliability of the system in the presence of multipath fading and interference, the method is based on arranging the frequencies available for transmission in a plurality of groups of frequencies, wherein each said group consists of a predetermined number of frequencies selected in such a way that they are approximately uniformly distributed in the entire available spectrum and separated by large but uneven frequency intervals, wherein a single message is transmitted on one or more frequencies in one group and subsequent messages are transmitted on the next frequencies in the group until all frequencies in the group are used, then a new group is selected and subsequent messages are transmitted using the frequencies from the new group and so on until all frequencies in all groups are used. Then, the process is repeated. Wherein the order in which the groups and the frequencies in the groups are selected is determined in accordance with a predetermined algorithm.
According to the third aspect of the invention, there is provided a method of minimizing the effect of collisions, the method is based on selecting the sequence to use the frequencies within each group and selecting the sequence in which the groups are used to be different for each transmitter, wherein resulting transmitter frequency sequence depends on the transmitter ID number or other number which is included in the transmitted message, so that, upon reception of a message from a transmitter, the receiver can determine what is the next frequency for this transmitter.
According to the fourth aspect of this invention, there is provided another method of minimizing the effect of collisions that can be used alone or in conjunction with the third aspect of this invention, the method comprising randomizing the time interval between transmissions individually for each transmitter and a receiver compensating for the time interval changes.
According to the fifth aspect of this invention, there is provided a method that allows such a system to convey the information about an abnormal sensor condition as soon as the condition occurs regardless of the transmission period of the associated transmitter. The method comprises of selecting an alarm frequency or preferably a group of alarm frequencies common for all transmitters. The alarm frequencies are used by the transmitters when an alarm or an abnormal sensor condition occurs, wherein when such a condition occurs in a transmitter, the transmitter transmits the messages sequentially on the alarm frequencies for a predetermined period of time after which the transmitter resumes transmissions according to the sequence before the alarm condition, wherein the receiver monitors the alarm frequencies during the time between the reception of scheduled messages from the transmitters.
According to the sixth aspect of this invention there is provided a method that allows the receiver to verify quickly whether the received message belongs to one of the transmitters associated with this receiver or some other spurious source without waiting for a complete message transmission. The method is based on encoding the transmitted pattern by interleaving the transmitted data with a predetermined pattern that can be decoded by the receiver without the reception of the entire message. Equivalently, the receiver can monitor other unique features of the received signal, for example modulation index or format, to accomplish that.